SYDNEY (Reuters) - The 1,000th whale shark, a rare and threatened species, has been discovered by researchers using a global program in which eco-tourists and scientists identify new sharks and lodge photographs on an online library.

"It's a major milestone, for science and for conservation," said ECOCEAN project leader Brad Norman in Australia.

"It was achieved with the help of ordinary people worldwide who want to study and protect this wonderful creature," Norman told Reuters on Thursday from Perth in Western Australia.

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The whale shark is the world's largest fish, a slow-moving filter feeder that can grow to around 12 meters (40 feet) and weigh up to 21 metric tons. But it is difficult to study, remaining in deep ocean for months and only rarely rising towards the surface.

Researchers and eco-tourists submit images, which are logged to reveal a picture of whale shark movements and behavior.

The 1,000th whale shark, a 6.5 meter (19 feet) male, was recently reported by marine biologist Simon Piercea in Mozambique. Piercea has contributed more than 100 sharks from his three-year study in Mozambique.

"We can expect there to be substantially more than 1,000 sharks alive in the world today," said Norman. "But, even so, it is still a very tiny global population that needs close monitoring to ensure its survival."

Participation in the ECOCEAN library has increased dramatically in recent years. It took three years to identify the 500th shark but only one additional year to reach 1,000.

ECOCEAN's Web site tells readers how to photograph a whale shark, warning swimmers to stay at least three meters (10 feet) from the shark for fear of upsetting it.

It also explains how to photograph the left and right side spot patterns above the pectoral fins, which create a unique "bodyprint," for identification.

"We're calling on the public worldwide to become 'citizen scientists' and help us study this wonderful animal by logging their images and sighting details," said Norman.

"The data will help us determine their numbers, movements and identify critical breeding and feeding grounds which need to be protected. This will build a better understanding of this threatened species and help save the largest fish in the ocean from extinction," he said.